Plastic bags having zippers and used for storing food and other items present unique manufacturing challenges because they have so-called “integral profiles” which are sections of the bag films which are thicker than other sections of the bag films. Zipper bags are generally manufactured by ether separately forming a bag body and zipper profile and appending the zipper profile to the bag body or forming the bag body and zipper profile together in one process. Extrusion is one method of forming a bag body and zipper profile in a single process. When a bag body and zipper profile are extruded together in the same process, the thicker sections that define the zipper profile cool more slowly than the thinner sections that define the bag body. The differential cooling rates impart stresses and can result in distortion of the profile shapes. Also, slower cooling rates can permit relaxation of shape which manifests itself as distortion. In order to maintain dimensional integrity of the thicker profile, slower cooling segments, as a general proposition it is necessary to use slow forming speeds so that the thicker profile sections are sufficiently hardened before the films are collapsed to minimize damage and distortion.
U.S. Pat. No. 3,875,281 describes generally some of the challenges in manufacturing blown films with integral profiles, such as distortion, closing of gaps, and manufacturing speed. The '281 patent proposes to address differential profile cooling challenges by establishing two control zones. Zone 1 is a region between the die head and the frost line and zone 2 is the region above the frost line. In zone 1, air is moved rapidly along the sides of the film to create a venturi effect to lower the atmospheric pressure. The arrangement requires a lower air ring 38 and shield 48, and in fact profiles were unsatisfactory in the absence of shield 48.